Two years with comet 67P/Churyumov-Gerasimenko: H2O, CO2, and CO as seen by the ROSINA/RTOF instrument of Rosetta

M Hoang,A Jäckel,J Lasue,H Gourlaouen,H Balsiger,U Mall,A Korth,H Rème,J H Waite,B Fiethe,Arnaud Beth ,André Galli ,Chia-Yu Tzou ,Léna Le Roy ,Thierry Sémon ,Kathrin Altwegg ,Philippe Garnier ,Martin Rubin ,Sébastien Gasc ,Ursina Calmonte ,Peter Würz

doi:10.1051/0004-6361/201834226

Abstract

Context. The ESA Rosetta mission investigated the environment of comet 67P/Churyumov-Gerasimenko (hereafter 67P) from August 2014 to September 2016. One of the experiments on board the spacecraft, the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) included a COmet Pressure Sensor (COPS) and two mass spectrometers to analyze the composition of neutrals and ions, the Reflectron-type Time-Of-Flight mass spectrometer (RTOF), and the Double Focusing Mass Spectrometer (DFMS). Aims. RTOF species detections cover the whole mission. This allows us to study the seasonal evolution of the main volatiles (H2O, CO2, and CO) and their spatial distributions. Methods. We studied the RTOF dataset during the two-year long comet escort phase focusing on the study of H2O, CO2, and CO. We also present the detection by RTOF of O2, the fourth main volatile recorded in the coma of 67P. This work includes the calibration of spectra and the analysis of the signature of the four volatiles. We present the analysis of the dynamics of the main volatiles and visualize the distribution by projecting our results onto the surface of the nucleus. The temporal and spatial heterogeneities of H2O, CO2, and CO are studied over the two years of mission, but the O2 is only studied over a two-month period. Results. The global outgassing evolution follows the expected asymmetry with respect to perihelion. The CO/CO2 ratio is not constant through the mission, even though both species appear to originate from the same regions of the nucleus. The outgassing of CO2 and CO was more pronounced in the southern than in the northern hemisphere, except for the time from August to October 2014. We provide a new and independent estimate of the relative abundance of O2. Conclusions. We show evidence of a change in molecular ratios throughout the mission. We observe a clear north-south dichotomy in the coma composition, suggesting a composition dichotomy between the outgassing layers of the two hemispheres. Our work indicates that CO2 and CO are located on the surface of the southern hemisphere as a result of the strong erosion during the previous perihelion. We also report a cyclic occurrence of CO and CO2 detections in the northern hemisphere. We discuss two scenarios: devolatilization of transported wet dust grains from south to north, and different stratigraphy for the upper layers of the cometary nucleus between the two hemispheres.

Highlights

The European Space Agency (ESA) Rosetta mission became the first spacecraft that followed a comet along its path around the Sun, and it was the first mission to achieve two landings on a comet
The upper panel shows that Reflectron-type Time-Of-Flight mass spectrometer (RTOF) did not continuously record data during the mission
We analyzed the full ROSINA/RTOF dataset during the Rosetta mission, from September 2014 until September 2016, for the three main volatiles H2O, CO2, and CO

Summary

Introduction

The ESA Rosetta mission became the first spacecraft that followed a comet along its path around the Sun, and it was the first mission to achieve two landings on a comet. In addition to these technical advances, the 11 instruments on board Rosetta collected data during more than two years. Massironi et al (2015) identified incompatible strata envelopes on the small and large lobes, suggesting that the bilobed shape is the result of a smooth collision between two independent bodies.

Objectives

Methods

Results

Discussion

Conclusion